1. Field of the Invention
The present invention relates generally to an information recording method, an information recording apparatus, and an optical information recording medium. More particularly, the present invention relates to an information recording method suitable for recording information on a phase change optical information recording medium such as a CD-RW, a DVD-RAM, a DVD-RW, or a DVD+RW using an information recording apparatus.
2. Description of the Related Art
In recent years, there has been a growing demand for high speed recording on an optical information recording medium. In turn, technologies for increasing the recording speed of the disk type optical information recording medium are rapidly developing since the recording/reproducing speed in this type of optical information recording medium can be increased by simply increasing its rotational speed. Particularly, an optical disk (disk type optical information recording medium) that can record information simply through intensity modulation of light that is irradiated upon recording is becoming increasingly popular these days. The simplicity of the recording mechanism of this type of optical disk enables a reduction of cost of the recording medium as well as the recording device. Also, since the intensity-modulated light is also used in the reproduction of the information, excellent compatibility can be realized with a reproducing-only apparatus. With the increase in the capacity of electronic information in recent years, there is presently an even greater demand for higher density and higher speed in the information recording technology.
The above described optical disk that is further characterized by using a phase change material is becoming the mainstream optical recording medium since it can be rewritten numerous times. In the optical disk using the phase change material, recording is performed by modulating the intensity of the irradiated light beam and creating a rapidly cooled state and a slowly cooled state in a recording layer material. When the recording layer material is rapidly cooled, an amorphous state is created, and when it is cooled slowly, a crystalline state is created. Thus, optical information can be recorded owing to the difference in optic physical properties between the amorphous state and the crystalline state.
The recording principle of the above optical disk uses complicated mechanisms of ‘rapid cooling’ and ‘slow cooling’ of the recording layer material. Thus, in high speed recording, the recording light undergoes a pulse division and a three-level intensity modulation to then be irradiated onto the recording medium. For example, this recording method is disclosed in Japanese Patent Laid-Open Publication No. 9-219021, Japanese Patent Laid-Open Publication No. 9-138947, Recordable Compact Disc Systems Part III (“Orange Book Part III”) version 2.0, Recordable Compact Disc Systems Part III (“Orange Book Part III”) Volume 2 version 1.1, and DVD+RW Basic Format Specifications version 1.1.
FIGS. 1A-1D are diagrams for illustrating the above recording method, wherein a mark shown in FIG. 1A is turned into data as shown in FIG. 1B where marked portions correspond to ‘High’ and unmarked portions correspond to ‘Low’. The above recording method is suitable for use in mark length recording or mark space recording. The mark has a temporal length that is an integer multiple of a basic clock period T. That is, the mark to be recorded has a temporal length nT where n is a natural number. The value range for the natural number n varies depending on the modulation method. In a compact disk system, n is within a range of 3-11. In a DVD system, n may take a value within the range of 3-11 or 14. In this drawing, n is set to n=6.
In the above described prior art, in order to form a mark with a temporal length of nT, an m number of multi pulses are irradiated as shown in FIG. 1C. The number m depends on the value of n, and their relationship is either m=n−1 or m=n−2. This is because the minimum value of n in a CD or DVD is 3. Also, an irradiation period of the pulse, that is, the rise period of each pulse is 1T, as shown in FIG. 1C where m=n−1, and in FIG. 1D where m=n−2. However, in either case, the period and width of a first pulse is independently set.
This recording method is characterized in that an increase of 1T in the mark length can be accommodated simply by adding one more pulse, and is thus very suitable for mark length recording.
However, when the recording speed is increased, the basic clock frequency increases. For example, in a 24× high-speed CD-RW, the basic clock frequency is 104 MHz, and in a 5× high-speed DVD-RW or DVD+RW, the basic clock frequency is 131 MHz. Thus, when recording is performed according to the conventional recording method (recording strategy) in these cases, the rise time and fall time of the pulse will take up a large portion of the total pulse irradiation time thereby decreasing the effective irradiation light energy, namely, the integration value.
FIGS. 2A-2C are exemplary diagrams illustrating the above effect. In these drawings, the dotted lines show the ideal irradiation waveforms and the solid lines show the actual light emission waveforms. In FIG. 2A, the actual light emission waveform is not rectangular as indicated by the dotted lines because of the time required for the rise and fall of the pulse. Thus, the pulse has a waveform as indicated by the solid line. When the basic clock is sped up further so that the rise time and fall time take up an even larger portion of the total irradiation time in the basic clock period, the irradiated pulse will be unable to reach a sufficiently high peak power Pw and a sufficiently low bottom power Pb as shown in FIG. 2B. That is, the peak power Pw will be Δ Pw lower and the bottom power Pb will be Δ Pb higher than the desired level. When the peak power Pw is lowered, there will be a decrease in the volume of material that can rise in temperature to a level sufficient for the material to turn amorphous. Also, when the bottom power Pb is not low enough, rapid cooling will be hampered thereby causing a re-crystallization of the material. This causes a decrease in the reproduction signal amplitude leading to a significant degradation of the reproduction reliability.
In order to solve the above described problem, a light source (laser diode and its drive unit) that can realize light emission with a short rise time and fall time is needed. However, to effectively function with a frequency above 100 MHz, the rise time and fall time need to be below 1 ns, which is very difficult to realize with the present technology.
Thus, in Japanese Patent Laid-Open Publication No. 9-134525 and in U.S. patent application Ser. No. 5732062, a technology for high speed recording using the conventional light emission source is disclosed. According to these prior art inventions, the number of irradiated recording pulses are reduced so that the mark having a length that is n times the basic clock period T, that is, the mark with a temporal length of nT, is formed through irradiation of m pulses where n=2m when n=even number, and where n=2m+1 when n=odd number, as opposed to the conventional art where n−1 pulses are irradiated for the same mark. For example, in a CD-RW that uses the EFM modulation (Eight to Fourteen Modulation; 8-14 modulation), n is a natural number within a range of 3-11. Thereby, in the conventional art when n=3, 4, 5, 6, 7, 8, 9, 10, and 11, the corresponding irradiation pulse numbers are: 2, 3, 4, 5, 6, 7, 8, 9, and 10, respectively. On the other hand, according to the methods disclosed in Japanese Patent Laid-Open Publication No. 9-134525 and U.S. patent application Ser. No. 5,732,062, when n=3, 4, 5, 6, 7, 8, 9, 10, and 11, the corresponding irradiation pulse numbers are: 1, 2, 2, 3, 3, 4, 4, 5, and 5, respectively. In this way, the pulse number can be reduced approximately by a half of the number of pulses used in the conventional art. Accordingly, the irradiation time of one pulse changes from 0.5T for the irradiation of n−1 pulses to 1T, which is double the conventional irradiation time, so that influence from the rise time and fall time can be reduced.
On the other hand, since the same number of pulses (m pulses) are irradiated to form recording marks with differing lengths 2mT and (2m+1)T, the irradiation period cannot be fixed. That is, when forming a recording mark with a length nT when n=2m, an irradiation time (the time when P=Pw) and a cooling time (the time when P=Pb) of a given pulse has to be made shorter compared to a case in which a recording mark with the length nT when n=2m+1 is recorded.
In Japanese Patent Laid-Open Publication No. 2001-331936, a recording method using an m number of multi-pulses for forming a recording mark with a temporal length of nT wherein n/m≧1.25 is disclosed. As in the above Japanese Patent Laid-Open Publication No. 9-134525, this patent application also describes the technology for recording marks with differing temporal lengths nT both when n=2m and n=2m+1 by irradiating the same number of pulses (m pulses). Herein, the irradiation time of the pulse is adjusted by modifying the irradiation time and cooling time of the first pulse and last pulse.
However, basically, according to the above methods, the irradiation time and the cooling time of all the pulses for each of the various mark lengths have to be defined. In turn, 69 parameters will be needed in the EFM (8-14 modulation) that is used in a compact disk and 77 parameters will be needed in a EFM+ (one type of the 8-14 modulation) used in a DVD. Thus, various techniques for reducing the number of parameters needed for defining the pulses are being proposed. For example, the irradiation time of a first pulse when m≧3 can be made to conform to a uniform length instead of being based on n, or the irradiation time and the cooling time of the middle pulses (the pulses other than the first and last pulses) can be made to conform. However, in the above examples, when m=1 or 2, that is, when n≦5, the parameters have to be set individually for each case. Therefore, a very large number of parameters will still be needed for defining the recording light emission waveform (recording strategy). Further, when the recording speed (scanning velocity) varies, a different recording pattern is needed for each recording speed. In such case, the irradiation time when P=Pw (i.e. the actual time of the pulse width as opposed to the relative time with respect to the clock period that can change depending on the recording speed) can be made to have a uniform length regardless of the recording speed.
Also, in a WORM (write once, read many) optical disk or a rewritable optical disk as represented by the CD-R/RW or the DVD+R/RW, parameters relating to the recording conditions of the disk are normally preformatted on the disk itself. For example, the preformatted disk information may be in the form of ATIP (Absolute Time in Pregroove) Extra Information in a CD-R/RW, or ADIP (Address in Pregroove) Physical Information in a DVD+R/RW. The preformatted disk information includes basic features such as the type of disk and the version of the disk standard, parameters needed for calculating the recordable scanning velocity and the optimum recording power in a test recording, and parameters that specify the optimum recording strategy. As for the parameters that specify the optimum recording strategy, there are ε (=Pe/Pw), and Strategy Optimization (dTtop, dTera) according to CD-RW standard specifications, and Ttop, dTtop, Tmp, dTera, ε1, ε2, according to DVD+RW standard specifications.
The information recording apparatus reads the above information upon recording information on a disk, and determines the recording strategy. Thus, it is preferable that detailed parameters be provided so that the recording device can determine an accurate recording strategy. However, detailed parameters will lead to an increase in information capacity requirements. Particularly, in a CD-R/RW system, the information capacity for recording the preformatted information is limited and in the case of a CD-RW, information worth 21 bits×6=126 bits is the maximum capacity for the preformatted information. To pre-format additional information, an area has to be newly defined in an unused area in either the outermost portion or the innermost portion of the disk such as the XAA (extra additional information area) in a multi-speed CD-R, or otherwise, the information has to be recorded using a pre-pit and the like.
As described above, the recording device reads the preformatted disk information upon recording information on the disk and sets the optimum recording strategy. When each disk has a large amount of parameter information, the processing of the information content becomes complex thereby causing the strategy generation circuit to be complicated.
Also, as mentioned earlier, it is preferable that the pulse irradiation time be arranged to be uniform. However, since marks with different lengths 2mT and (2m+1)T are recorded by irradiating the same number of m pulses, it is impossible to fix the irradiation period to a uniform time period. In the above case, when the irradiation period of a mark with length nT (where n=2m or n=2m+1) is set according to the value of n, the strategy generation circuit will be very complicated. That is, the irradiation period will have to be set individually for each case, and when the irradiation pulse timing is set individually as opposed to being in accordance with the basic clock timing, the circuit design becomes extremely complicated.
Also, under the restriction of having to record marks having differing lengths 2m and 2m+1 with the same number of m pulses, if the method according to the so-called “Orange Book Part III” is used, wherein only the pulse width of the last pulse is adjusted when the value n of the mark with length nT is an odd number, the difference in the irradiation between the last pulse and the rest of the pulses will be too distinct and the mark formed will not have a consistent shape (the mark corresponding to the last pulse is likely to become larger). As a result, reproduction signals for this recording mark will have a distorted waveform, causing an increase in the generation of jitters.
Also, for the reasons described above, the determination of the recording strategy is preferably realized with few parameters but with accuracy.